1. Field of the Invention
The present invention relates to a DC current sensor used in a DC leakage breaker and the like, and, particularly, to a sensitive DC current sensor having a relatively simple construction and a good detecting capability for very small current variations.
2. Description of the Prior Art
In recent years, though equipments using a DC current such as electrical appliances incorporating an inverter, electromobiles and the like are increased, requirements on a sensor which detects loads of DC motors built in the various equipments for necessary control, and a DC current sensor used in a DC leakage breaker and the like are enhanced.
As the DC current sensor used in an AC leakage breaker and the like, those applying a current transformer is known widely.
However, this configuration can not be adopted in the leakage breaker used in the aforementioned equipments using the DC current, and hence, conventionally, a shunt resistance method, a magnetic amplifier method, a magnetic multi-vibrator method (Japanese Patent Application Laid Open Nos. Sho 47-1644, Sho 53-31176, Sho 59-46859), a hall device method and the like have been studies.
The shunt resistance method is a method detecting a potential difference generated at opposite end portions of the shunt resistance, by arranging the shunt resistance in a wire being detected in series.
In the magnetic amplifier method and the magnetic multi-vibrator method, a core made of a soft magnetic material consisting of a detecting coil wound in a toroidal shape is used, whereby the wire being detected is extended through the core for DC magnetic deflection of the core made of the soft magnetic material, by the DC current flowing through the wire being detected within a saturated magnetic flux density (Bs), thereby imbalance is produced on a time for alternating magnetic flux generated by applying the AC current to a coil wound on the core in advance, to reach saturation in the positive and negative directions to detect the variation by the detecting coil. Since the magnetic flux variation is given in the core in advance in the magnetic amplifier method, though a configuration of winding an exciting coil on the core to apply the AC current of a predetermined value is adopted, in the magnetic multi-vibrator method, it is so constituted that, the self-oscillation takes place by the operation of a semiconductor and the like in a circuit connected to the detecting coil, and a duty ratio of the oscillation wave form is changed responsive to the electric current being detected for oscillation.
Furthermore, the hall device method is constituted such that, the wire being detected is wound in a toroidal shape directly around the core made of the soft magnetic material, whereon a gap portion for arranging the hall device is formed partially, and the magnetic flux variation in the core in response to the variation in the DC current flowing through the wire being detected is detected.
However, the DC current sensor consisting of the above-mentioned methods are difficult to respond to a very small current variation of the DC leakage breaker and the like by the following reasons, and at present, they are not practically used as the sensitive DC current sensor.
That is, in the shunt resistance method, since the shunt resistance itself is arranged, as an electrical resistance, in a circuit including the wire being detected, an electrical loss in the circuit increases and an electrical effect is bad.
Also, since a detecting circuit for detecting the potential difference produced at opposite ends of the electrical resistance is directly connected to the wire being detected, the detecting circuit and the wire being detected are difficult to be insulated electrically, and for example, the detecting circuit and an applied circuit such as a microcomputer control circuit can not be connected directly, results in a poor versatility.
Besides these disadvantages, though two shunt resistances must be arranged in the circuit of the wire being detected in order to adopt the shunt resistance method in the leakage breaker, it is, in fact, difficult to make characteristics of the shunt resistances uniform, thus a precise measurement of the potential difference can not be realized.
Besides, in order to detect a small leakage by comparing the potential difference measured by the detecting circuit connected to the respective shunt resistances, it is necessary to connect the respective detecting circuits by a very complicated electric circuit, thus it is difficult to provide as the DC current sensor of high practicability.
In the magnetic amplifier method and the magnetic multi-vibrator method, though the detecting circuit and the wire being detected can be insulated electrically, as previously described, the core made of the soft magnetic material must be magnetically deflected by the DC current flowing through the wire being detected, so as to be saturated, substantially near to the saturated magnetic flux density (Bs). When using the known soft magnetic material such a permalloy as the core, for example, when the electric current flowing through the wire being detected is about several tens mA, the wire being detected must be wound around the soft magnetic material core by several tens to several hundreds turns or more, thus, originally, it was difficult to be used as the DC current sensor for the leakage breaker and the like requiring one through turn of the wire being detected.
Also, in the magnetic amplifier method and the magnetic multi-vibrator method, it was difficult to detect a directivity of the DC current flowing through the wire being detected.
Also, in the hall device method, since the detecting capability is inevitably decided by characteristics of the hall device, when the hall device known at present is used, for example, when the electric current flowing through the wire being detected is about several tens mA, the wire being detected must be wound around the soft magnetic material core over several tens to several hundreds turns, thus, similar to the magnetic amplifier method and the magnetic multi-vibrator method, it was difficult to be used as the DC current sensor for the leakage beaker and the like requiring one through turn of the wire being detected.